As deeper ocean depths are developed, the need to lower as well as lift such items as wellheads, piles, tools, and pipeline systems great distances arises. Because of such depths, the speed of the object being lowered or lifted as well as its weight becomes important.
In the past and at lesser depths, barge mounted heavy lift cranes were used because they had acceptable weight capacity and speed for such distances. At greater depths, however, their many parted lines caused tangling problems and providing sufficient storage capacity for the wire rope also presented its problems.
Derrick mounted crown and traveling blocks have been used to repeatedly add/remove segments of drilling pipe to accomplish a lowering/lifting operation. This equipment has adequate weight capacity but the repetitive task of constantly adding a length of pipe or constantly removing a length of pipe, and its associated work stoppages, results in an undesirable speed rate for deepwater operations.
Another method of providing lowering/lifting capability involves the use of a multi-part underwater traveling block arrangement. Such system requires either larger hoists, more winches, or a reduction in the reeving of the main block. The installation of additional equipment is often not an option due to existing equipment congestion, and such re-reeving will reduce the lift capacity in addition to being a time-consuming task. Furthermore, a submersible block is subject to catastrophic twisting, due to reduced torque resistance, which can generate friction sufficient enough to preclude traveling block retrieval.
Fixed and traveling cable grip assemblies provide acceptable weight capacities and have the simplicity of requiring few lengths of large diameter wire rope. However, the short stroke length and slow speed at which the grips move along the cable produces a very slow lowering/lifting speed. Furthermore as water depth increases, this slower speed only increases the length of offshore time required.
It is thus an object of this invention to provide a lowering/lifting system which meets and/or exceeds required weight capacity, is simple to operate, and requires few parts of wire rope. Further objectives of this invention are to enable its operation whether it is crane mounted, deck mounted, or derrick mounted, and to maximize the use of existing installed equipment, machinery, foundations, and structural load paths. Another object is to significantly increase the lowering/lifting speed of the load. Still yet a further object is to reduce the number of parts of line normally required for greater depths so as to minimize the potential of tangled or twisted rope. These and other objects and features of this invention will become apparent upon closer investigation of this invention.